Abstract : It has recently been recognized that single intensely-ionizing particles can cause soft upsets in microelectronic components used in spacecraft. These particles can be heavy ions in galactic cosmic rays or heavy ions accelerated in solar flares. When the upsets are due to the trapped radiation, they are thought to result from nuclear interactions initiated by trapped protons. In this report, we have investigated whether there are sufficient numbers of heavy ions trapped in the Van Allen belts to be the dominant cause of soft upsets. We conclude, based on the scanty data available, that the contribution from upsets caused by trapped heavy ions is likely to be important at least under some conditions. We also found that we could not rule out the possibility that trapped heavy ions may cause upset rates that are orders of magnitude larger than those predicted from the trapped proton population. (Author)